Google Earth

Google Earth is a free software tool that "lets you fly anywhere on Earth to view satellite imagery, maps, terrain, 3D buildings and even explore galaxies in the Sky. You can explore rich geographical content, save your toured places and share with others" (from the Web site). It is an amazing view of the Earth that allows the user to show/hide various features including political borders, roads, nature preserves, and even 3-D views of buildings. You can type in a location (address, city, etc.) to go directly there or drag your mouse around and double-click a spot to zoom in.

Google Earth is a great tool for a number of different types of exploration in different content areas. You'll see from the samples linked below that GE can be used in many different disciplines in both teacher- and student-directed ways. Entire learning communities are popping up on the Web so that teachers using GE can connect with one another, share ideas and resources, and post student work. To explore GE, I'd suggest you spend a few minutes playing with the software and try out different features (e.g. going to a specific location, browsing, selecting/deselecting different features, etc.). Once you have a basic feel for how it works, check out the examples below.

Samples: (note that if there is a link to a .kmz file, this is a file that will open up in Google Earth)